In a three phase systems, the use of transformers with three windings is common.  These windings are often connected in delta or star, resulting in common transformer configurations such as delta-delta or delta-star.  An open delta transformer or also known as V-V transformer uses two single phase transformer or two windings of a 3 limbs transformer to provide either a three-phase or single phase supply to the load.

While open delta transformer is cheaper than a conventional three winding transformer, the open delta will only deliver 57.7% of the power of a conventional transformer not two thirds, 66.7% as may be expected.  There is limited adoption of open delta transformers although they can be useful in certain situations such as when a 3 phase to single phase system is required or during emergency condition where one of the 3 single phase transformers of a 3 phase system is faulty.